This invention relates to a filter screen exchanging apparatus for a plastic extruder.
Screen exchanging devices are employed to replace a contaminated filter screen with a new screen in a plastic extruding apparatus without stopping the extrusion operation when a screen becomes so contaminated with foreign objects entrained in the resin that its openings are clogged. The rotary valve type screen exchanging device of the prior art is so constructed that the flow of plastic resin material is switched over by a valve to exchange a filter screen. In this device, however, the characteristics of the plastic resin deteriorate due to stagnation since the flow passage is complicated. Further, since the change-over valve must be rotated under considerable back pressure, great operating force is required. Such a device is only usable with small sized extruders.
A conventional slide type screen exchanging device is shown in FIGS. 1 and 2, wherein a slide plate 2 is perpendicularly disposed in a plastic extruding apparatus 1 and a pair of breaker plates 3 and screens 4 are mounted on the slide plate. The screens 4 are mutually displaced and exchanged in a longitudinal direction by hydraulic cylinders (not shown), and are secured by snap rings 5. In FIG. 2, as a space A near the tip of the extruding screw is filled with plastic resin during operation, and when the slide plate 2 is moved to implement screen exchange, the new screen 4 is liable to be stripped off or bent away due to the pressure of the plastic resin at space B. As a result some unfiltered plastic resin may appear in the final product. Further, in such an exchanging device it is necessary to provide some clearance space C between the outermost surface of the slide plate and the screen 4, whereby air may be introduced into the extruded resin and or some resin may escape to the outside through the space C during the movement of the slide plate 2. This abruptly reduces the pressure of the resin, which takes a long time to restore to its normal level, and such non-uniform extrusion results in defective products.
Furthermore, in a large capacity extruding apparatus the disc-shaped breaker plate must be relatively thick to provide sufficient mechanical strength, which greatly reduces the resin pressure. When used screens are exchanged for new ones the breaker plates must also be exchanged since if they are left exposed to the air the resin adhering to them deteriorates. Recently, large capacity extruding apparatuses have been developed using a breaker plate weighing 30 kg or more and having 1000 flow nozzles therein each 50 mm long and 7 mm in diameter. The diameter of the extruder screw used in such an apparatus is 300 mm. Obviously, screen exchange and breaker plate cleaning in such an apparatus is extremely difficult and costly.